Condensed oximeter system and method with noise reduction software

ABSTRACT

A compact pulse oximetry system and method which separates the combined signal into its respective AC and DC components. By separating the signal into AC and DC components, a smaller order bit A/D converter may be used while still maintaining signal accuracy. Instead of using the combined signal to calculate the oxygen saturation content, the system microprocessor computes the Ratio of Ratios using the derivative of the separated AC component of the diffused signal to calculate the oxygen saturation of the measured fluid. To calculate the Ratio of Ratios, a ratio of the derivative value of the separated AC component is used. Instead of taking a single sample between the peak and valley of the signal, the oximeter system samples each value. To decrease the effect of system noise, a linear regression is performed over each sample.

RELATED APPLICATION DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 08/447,665, filed on May 23, 1995, now U.S. Pat.No. 5,577,500, which is a continuation application of U.S. patentapplication Ser. No. 08/225,486, filed on Apr. 8, 1994, now U.S. Pat.No. 5,533,507, which is a continuation application of U.S. patentapplication Ser. No. 07/740,362, filed on Aug. 5, 1991, now U.S. Pat.No. 5,351,685.

BACKGROUND OF THE INVENTION

Certain components in the blood absorb light more strongly at differentwavelengths. For example, oxyhemoglobin absorbs light more strongly inthe infrared region then in the red region. Therefore, highly oxygenatedblood having a high concentration of oxyhemoglobin will tend to have ahigh ratio of optical transmissivity in the infrared region. The ratioof transmissivities of the blood at red and infrared wavelengths can beemployed in calculating oxygen saturation of the blood.

This principle has been used in oximeters for monitoring oxygensaturation of the blood, as for example, in patients undergoing surgery.Oximeters for this purpose may include a red and infrared light emittingdiode together with a photodetector. The oximeter is typically clampedto an appendage of the patient's body, such as an ear or finger. Theoximeter directs a beam of red and infrared light of known frequency andwavelength into the appendage. A sensor on the other end of theappendage receives the diffused light. Knowing the change in wavelength,frequency, and intensity of the diffused light beam, the oximeter canquickly determine the oxygen saturation level of the patient.

The diffused light signal received by the photodetector is an analogsignal which includes both an AC and DC component. The diffused lightsignal includes an AC component which reflects the varying opticalabsorption of the blood due to variance in the volume of the blood dueto the pulsatile flow of blood in the body. The diffused signal alsoincludes an invariant or DC component related to other absorption, suchas absorption by tissues other than blood in the body structure.

The diffused analog light signal is converted into a digital signal. Thedigital representation is used by the oximeter system microprocessor forprocessing the oxygen saturation level. Because calculation of theoxygen saturation is critical for determining the status of the patient,a high degree of accuracy is required for the digital representation ofthe diffused analog light signal. A problem in converting from an analogto a digital representation of the signal is that the DC component is somuch larger than the AC component of the diffused signal. To encompassboth the AC and DC components of the entire diffused signal requiresusing a 16 bit A/D converter. Using a smaller A/D converter, for examplean 8 bit A/D converter, would cut off the least significant bits of thesignal, namely the AC component of the signal. Receiving an accuraterepresentation of the AC component is critical, since it is the ACcomponent of the diffused signal which reflects the oxygen absorption.

Because of the degree of precision necessary to accurately reflect theAC component of the signal, currently available oximeter systemstypically uses a 16-bit A/D converter. A 16 bit A/D converter may befour times as expensive as currently available microprocessors, such asthe manufacturer name(80C196K)!, which include an 8 bit A/D converterand a pulse width modulator in a single chip. Since 8 bit does not givea sufficient degree of accuracy to encompass for oxygen absorptionmeasurements, a combined microprocessor A/D converter chips such as themanufacturer name (80C196K)! cannot be used. A separate 16 bit A/Dconverter must be used.

In addition to increasing the costs of the oximeter system, using aseparate 16 bit A/D converter increases the size and power consumptionof the system. Adding a separate 16 bit A/D converter adds to the sizeof the measurement system. Because the oximeter measurement system isconnected to a patient and because the system is often moved betweenpatient rooms, compactness of size of the pulse oximeter measurementsystem is highly desirable.

The digital representation of the diffused signal is used by theoximeter system microprocessor to calculate oxygen saturation level inthe blood of a patient. The Ratio of Ratios, a variable used incalculating the oxygen saturation level, is typically calculated bytaking the natural logarithm of the ratio of the peak value of theinfrared signal divided by valley measurement of the red signal. Theaforementioned value is then divided by the natural logarithm of theratio of the peak value of the red signal divided by the value of valleymeasurement of the infrared signal.

The diffused signal is sampled several times during each period todetermine the peak and valley measurement for each period of thewaveform. In calculating the Ratio of Ratios, the peak value is assumedto be the high sample value during the period of the waveform. Thevalley measurement is assumed to be the low measured value. Althoughthis method leads to a good estimate of the variable R, taking the peakand valley measurements over the entire time interval is prone to errorsince the sampling is taken between a single pair of points. Thisignores variation in the signal between different pulses during themeasured time interval.

A problem with choosing a single peak and valley measurement during asampling interval, is corruption of the measurement by system noise. Forexample, patient motion of the oximeter during the sampling period maycause drift in the AC component of the signal. Also, ambient light orelectrical noise may increase system noise. If the added noise on thepulse creates a false peak or valley measurement during the samplinginterval, this will cause an incorrect value for the Ratio of Ratios. Aninexpensive, noise insensitive oximeter measurement system is needed.

SUMMARY OF THE INVENTION

The present invention provides a compact pulse oximeter probe whichincludes at least one narrow bandwidth light emitting diode and at leastone photoelectric sensor. The signal measured by the photoelectricsensor has both an AC and DC component. Instead of using a high order 16bit A/D converter to maintain signal accuracy, the present inventionseparates the AC and DC components and uses a smaller A/D converter.Instead of using the combined signal to calculate the oxygen saturationcontent, the system microprocessor computes the Ratio of Ratios usingthe derivative of the separated AC component of the diffused signal tocalculate the oxygen saturation of the measured fluid.

The AC component is separated out by using a capacitor which is normallycharged immediately after each valley of the AC sensor signal. Thevalley represents the DC portion of the AC sensor signal. By subtractingthe voltage on the capacitor from the combined signal, only the ACcomponent is left. A fixed DC offset is added to place the resulting ACsignal in the middle of the range of the A/D converter.

Instead of taking a single sample between the peak and valley of thesignal, the oximeter system samples multiple values. A derivative of theAC component is taken between each pair of sample values. This ACderivative value is used in calculating the Ratio of Ratios. Because theratio of the derivative of the red and infrared signals is a straightline, a linear regression may be performed to decrease the effect ofsystem noise.

The present invention also uses a pulse width modulator in themicroprocessor to control the peak-to-peak values of both the red andinfrared signals so that they extend across most of the range of the A/Dconverter to give maximum resolution. This can be done by controllingthe intensity of the LED driver so that the average signal has thedesired intensity. Because the red and infrared signals producedifferent levels of the detector signal, they are amplified differently.Alternatively, on the received end the signals can have theamplification factor controlled by the pulse width modulator.

By separating the AC and DC component of the diffused signal, a 16 bitA/D converter is not necessary to give an accurate representation of theAC component. Since a digital representation of the separated ACcomponent can be accurately represented by an 8 bit A/D converter, acombination microprocessor analog to digital converter chip may be used.Using a combination microprocessor decreases system size and cost.Furthermore, using a single chip microprocessor decreases overall powerconsumption of the system.

A further understanding of the nature and advantages of the inventionmay be realized by reference to the remaining portion of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the oximeter measurement system accordingto the present invention.

FIG. 2 (comprising FIGS. 2A-2H) is a circuit schematic of the oximetermeasurement system of FIG. 1.

FIG. 3 is a circuit schematic of the Current to Voltage Converteraccording to FIGS. 2A-2H.

FIG. 4 is a circuit schematic of the Zeroing Circuit of FIGS. 2A-2H.

FIG. 5 is a circuit schematic of the Demultiplexer of FIGS. 2A-2H.

FIG. 6 is a circuit schematic of a Variable Gain ControlledAmplification Unit of FIGS. 2A-2H.

FIG. 7 is a circuit schematic of the Filtering Circuit of FIGS. 2A-2H.

FIG. 8 is a circuit schematic of the Offset Subtractor Circuit of FIGS.2A-2H.

FIG. 9 is a circuit schematic of a Second Variable Gain ControlledAmplification Unit of FIGS. 2A-2H.

FIG. 10 is a circuit schematic of the LED Drive and Select Circuit ofFIGS. 2A-2H.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to an improved oximetry measurement systemfor measuring the blood constituents. The oximeter measurement systemincludes an oximeter optical probe typically comprised of two lightemitting diodes, a photoelectric sensor, and a means for fastening theoptical probe to an appendage. The first LED typically emits frequencyin the infrared range. The second LED typically emits light in the redrange of the spectrum.

The infrared and red light emitting diodes are switched on and off in analternating sequence at a switching frequency far greater than the pulsefrequency. The signal produced by the photodetector includes alternatingportions representing red and infrared passing through the bodystructure. These alternating portions are segregated by sampling devicesoperating in synchronism with the red/infrared switching, so as toprovide separate signals on separate channels representing the red andinfrared light transmission of the body structure.

The oximeter probe is typically clamped to an appendage of the patient'sbody, such as an ear or finger. For each heartbeat, fresh arterial bloodis pumped into the capillaries of the appendage. Light from the two LEDsis modulated by the pulsatile component of arterial blood therebycausing a periodic increase and decrease in the light intensity observedby the sensor. The oxygen saturation of hemoglobin in the pulsatileblood may be determined by the oximeter.

FIG. 1 illustrates a block diagram of the oximeter measurement systemaccording to the present invention. The oximeter measurement systemincludes a Current/Voltage Converter Unit 300, a Zeroing Unit 400, aDemultiplexer 500, a Variable Gain Amplifier Unit 600, a Filtering Unit700, an Offset Subtractor Circuit 800, a Second Variable Gain AmplifierUnit 900, and a Microprocessor 990. The output from the photoelectricsensor is fed into both the Current/Voltage Converter Unit 200 and theZeroing Unit 300. When an LED signal value is received, thephotoelectric output current is converted into a voltage by theCurrent/Voltage Converter Unit 200. When no LED signal value isreceived, the Zeroing Unit 300 zeroes the output of the Current/VoltageConverter Unit 200.

The infrared and red light emitting diodes are switched on and off in analternating sequence. These alternating portions must be segregated soas to provide separate signals on separate channels. A Demultiplexer 400selects whether the sensed current is a infrared signal or a red signal.Both the infrared and red signal are input into the Variable GainAmplification Unit 400. Amplifier 402 typically amplifies the infraredsignal. Amplifier 404 amplifies the red signal. Different amplificationfactors may be used. The amplification factors are controlled by a pulsewidth modulation signal from processor 990.

Both the infrared and red LED signals are fed into a Filtering Unit 600for reducing noise at the switching frequency. The filtered signal isfed into an Offset Subtractor Circuit 700. The Offset Subtractor Circuit700 separates the AC component of the combined signal from the combinedsignal. The separation is accomplished by tracking the AC component ofthe combined signal through a capacitor periodically charged to the DCvalue.

The AC component is passed through a Second Variable Gain AmplificationUnit. The AC component and combined AC and DC signal are amplified bydifferent amplification factors. The amplified signal is fed into aMicroprocessor Unit 990 which computes the oxygen saturation level inthe blood. Both the AC signal (lines 952, 953) and the combined AC andDC signal (lines 950, 951) are fed in analog form to processor 990,which uses its internal 10 bit A/D converter to convert them intodigital form. Calculating the oxygen saturation level is done by thesoftware in the microprocessor unit 990. The microprocessor uses thederivative of the separated AC component of the combined signal tocalculates the Ratio of Ratios. Knowing the Ratio of Ratios allows forcalculation of the oxygen saturation content.

The Current/voltage Divider

The oximeter probe photodetector 210 senses a current in response to theinfrared or red light source projected through an appendage. The sensedcurrent is transformed into a voltage via the Current to VoltageConverter 300. FIG. 3 is a circuit schematic of a Current to VoltageConverter 300 according to the present invention. The current on line302 from the photodetector 210 is input into a resistor capacitornetwork 301 comprised of resistor 302 and capacitor 308. The resistorcapacitor network 301 filters high frequency noise from thephotodetector signal.

The in put terminals of amplifier 320, capacitors 308, 314, resistors306 and 318 are all connected to node 310 which provides a virtualground. Thus when current flows across resistor 306, a voltage dropoccurs across capacitor 308 which filters the input signal. Currentflowing from resistor 306 across resistor 318 provides the current tovoltage conversion. Since node 310 acts as a virtual ground, currentflowing into the input terminal of amplifier 320 adjusts to take up anycurrent across resistor 318. The voltage drop across resistor 318 isoutput at node 316 of amplifier 320 and is proportional to the sensedcurrent.

The Zeroing Circuit

Current flowing from the output node 316 of the Current/VoltageConverter is input into the Zeroing Circuit 400. A circuit schematic ofthe Zeroing Circuit is illustrated in FIG. 4. The purpose of the ZeroingCircuit 400 is to prevent a voltage at output node 316 when therespective LED is off. Current flow in photodetector 210 may occur dueto residual light or from the LED circuit not being fully turned off.

The output signal on line 317 of the Current/Voltage Converter is inputto a switch 402. A second switch 406 is connected to switch 402. Bothswitches 402 and 406 are tied together so that they both close or opensimultaneously. FET switches 402 and 406 should both have their leadsaligned so that both of their drains, or alternatively both of theirsources, face amplifier 416. By positioning their leads in this manner,charge injection is minimized.

Both switches 402 and 406 are coupled to the input terminals of aninverting amplifier 416. When switches 402 and 406 are closed, acapacitor 424 begins charging to the output value of node 316 of theZeroing Circuit. Increasing the voltage at positive terminal 412 ofamplifier 415 results in an increased voltage at negative terminal 420.Creating a positive voltage on node 420 causes current to flow intocapacitor 418 and produces current flow through resistor 410, which isconnected to virtual ground 310.

Thus, when both switches 402, 406 are closed, a positive current flowsthrough resistor 412 causing current to into the virtual ground node310. This positive current flow results in a decrease in voltage onoutput node 316 of the Current/Voltage Converter 300. Thus when switches402, 406 are closed, a positive voltage on the output node 316 of theCurrent/Voltage Converter 300 generates a current which forces outputnode 316 of the amplifier 320 to zero.

When both switches 402, 406 are open, the voltage stored on node 422before opening of the switch will be held there by the capacitor 424.This voltage offset gets rid of the offset in the amplifier 416.Similarly since switch 406 is open, there is no discharge path for thecharge stored on node 420. The current going through resistor 412 willcontinue to flow at the level flowing immediately before the switch 406was opened. Thus the current which sets output node 316 to zero isinjected with the current coming in. So the output node 316 is going toreact as if there was no current coming in initially. The current goingthrough resistor 412 is going to cancel the ambient current.

The Demultiplexer

FIG. 5 is a circuit schematic of the Demultiplexer Block 500 accordingto the present invention. The Demultiplexer Block has two separatedemultiplexers 510 and 530 for the infrared and red signalsrespectively. The red and infrared demultiplexers are functionallyequivalent. The red signal demultiplexer 530 is described forillustration.

The red signal demultiplexer 530 is comprised of a switch 532, and aresistor 534 and capacitor 540 in series. The timing of the opening andclosing of switch 532 is controlled by the microprocessor. After the redsignal stabilizes switch 532 is closed allowing the red signal to passfrom node 316 to node 532. Series resistor 534 and capacitor 540 filterthe sampled red signal.

The Variable Gain Amplifier

FIG. 6 is a circuit schematic of a Variable Gain ControlledAmplification Unit 600 according to the present invention. The variablegain amplifier 600 amplifies the detected LED signals with the amount ofamplification controlled by the pulse width modulator 991 of themicroprocessor 990.

The Variable Gain Controlled Amplification Unit is comprised of a pairof amplifiers 602, 604 coupled to a switch 606. In the illustratedembodiment, amplifier 604 amplifies the red LED signal and amplifier 602amplifies the infrared LED signal. Both amplifiers are functionallyequivalent. The red variable gain amplifier is described forillustration purposes.

The output from both amplifiers 602, 604 are input into a switch 606.The switch 606 contains three single pole double throw switches. Theoutput of amplifier 604 is input into pole 608 and pole 610 of theswitch 606. Pole 608 is normally connected to ground. Pole 610 isnormally connected to the output of the amplifier 604. Thus the outputof amplifier 604 at node 612 is alternately switched between ground andthe value of the amplifier output.

The timing control of the switching is controlled by the pulse widthmodulator 991. The pulse width modulator signal is the input at node 616of switch 606. The pulse width modulator signal at node 616 controls howlong node 613 is connected to ground and the amplifier output.

Thus the pulse width modulator signal can be varied to effect how longthe pulse width is on or off. For example, if the pulse width modulatorsignal on line 614 is off 2/3 of the time and on 1/3 of the time, thesignal at node 613 would be connected to ground for 2/3 of the dutycycle of the pulse width modulator and connected to the output of theamplifier for 1/3 of the duty cycle of the pulse width modulator. Theswitch output signal at node 613 is input into a resistor capacitornetwork 620 comprised of a resistor 614 and a capacitor 618. Theresistor capacitor network 620 acts to average the switch output signalat node 613. The resistor capacitor network 620 produces an effectivevoltage which is 1/3 of the output of the operational amplifier 604. Theoutput 613 is the amplification factor for the amplifier 604.

Filtering Unit

The output signal from the Variable Gain Amplification Unit 600 isfiltered before being input into the Offset Subtractor Unit 800. FIG. 7is a circuit schematic of the Filtering Unit 700 according to thepresent invention. The filtering unit 700 consists of two parallel twostage second order filters. A first two stage filter 730 is dedicated tofiltering of the red LED signal; a second two stage filter 740 isdedicated to filtering the infrared LED signal. Both two stage filters730 and 740 are functionally equivalent. The red LED two stage filter730 is described for illustration purposes.

The output from amplifier 604 at node 612 is input into a first stagesecond order filter 702. The first stage filter is comprised of anamplifier 716, a series resistor 704 in parallel with a capacitor 708and a resistor capacitor network including resistor 710 and capacitor712. The resistor capacitor network is in parallel with the positiveinput terminal 714 of the inverting amplifier 716. The first stagefilter 702 filters the input stage to reduce noise at the switchingfrequency.

A second stage filter is similarly comprised of an amplifier 730, aseries resistor 270 in parallel with a capacitor 722 and a resistorcapacitor network including a resistor 724 and a capacitor 726. Theresistor capacitor network is in parallel with the positive inputterminal 728 of the inverting amplifier 730. Similar to the first stagefilter 702, the second stage filter 704 reduces noise at the switchingfrequency. In addition, a pair of diodes 732 and 734 are connected inparallel to the positive input terminal 728 of the inverting amplifier730. The diode pair acts to limit the output voltage of the invertingamplifier 730 so that it does not exceed the range of the analog todigital converter by more than 0.7 volts.

The Offset Subtractor Circuit

The Offset Subtractor Circuit eliminates the need for a 16 bit A/Dconverter by subtracting the DC offset from the combined waveform thusleaving the AC component of the waveform. FIG. 8 is a circuit schematicof the Offset Subtractor Circuit 800 according to the present invention.The Offset Subtractor Circuit 800 includes a first offset subtractorunit 802 for the subtraction of the DC component of the red LED signaland a second offset subtractor circuit 804 for subtraction of the DCcomponent of the infrared LED signal. The first and second offsetsubtractor circuits are functionally equivalent. The red LED offsetsubtractor circuit is discussed for illustration purposes.

The Offset Subtractor Circuit 810 is comprised of a capacitor 808, aswitch 810, and resistors 812 and 814. The combined waveform for the redLED signal is output from the filtering unit at node 816. The combinedsignal is split. The combined signal is input into the A/D converteralong line 818 and input into capacitor 808.

Resistors 812 and 814 act as a voltage divider and provides a baselinevoltage at node 822 which is approximately in the middle of the A/Dconverter range at 2.0 Volts in the absence of current through capacitor808. This voltage position is important since the analog component ofthe waveform will vary in both the positive and negative directionsaround the reference voltage. Node 822 is tied to a reference voltagethrough resistor 812. When switch 810 is closed, capacitor 808 ischarged to the voltage difference between node 820 and node 822. Thusthe voltage at node 822 is equal to the voltage across capacitor 810 isthe difference between the DC component and the baseline voltage.

When the switch 810 is opened, the voltage at node 822 is left at thedifferential voltage. Thus the voltage at node 824 will be equal to thecombined AC and DC voltage minus whatever the combined DC voltage was atthe instant the switch 810 was opened.

The microprocessor controls the switch timing so that the switch isclosed immediately after each valley of the detector signal. The amountof time the switch is closed and the values of the capacitor andresistors are chosen so that the capacitor can charge in the intervalthe switch is closed. The microprocessor knows not to sample the signalduring this interval.

The Second Variable Gain Amplifier

FIG. 9 is a circuit schematic of a Second Variable Gain ControlledAmplification Unit according to the present invention. The secondvariable gain amplification unit is comprised of an amplification unit902 for the red LED signal and a second amplification unit 904 for theamplification of the infrared LED signal. Both amplifiers arefunctionally equivalent. The red amplification unit 902 is described forillustration purposes.

The Second Variable Gain Controlled Amplification Unit 900 is similar tothe First Variable Gain Amplifier 600. First amplifier 600 amplified thecombined DC and AC signal to the optimum level for the range of the A/Dconverter while second amplifier 900 amplifies the AC signal to theoptimum level for the range of the A/D converter. The variable gainamplifier 900 amplifies the detected LED signals with the amount ofamplification controlled by the pulse width modulator 991. The redvariable gain amplification unit 902 is comprised of a pair ofamplifiers 903 and 905 coupled to a switch 906 and a pair of resistorcapacitor networks for filtering.

The output from both amplifiers 903 and 905 are input into a switch 906.The switch 906 contains three single pole double throw switches. Theoutput of amplifier 903 is coupled to pole 908. The output of amplifier905 is coupled to pole 910. In the First Variable Gain AmplificationUnit 600, the switch 606 sequences the amplifier 604 between ground andthe output of the amplifier. In the Second Variable Gain AmplificationUnit 900 the switch sequences the amplifier 905 between a referencevoltage of 2.0 volts and the output of the amplifier. The 2.0 voltreference voltage is buffered by the amplifier 903. Pole 908 is normallycoupled to a reference voltage. Pole 910 is normally coupled to theoutput of the amplifier. Thus the output at pole 912 is alternatelyswitched between ground and the amplifier output.

Switching between the output voltage of the amplifier and the referencevoltage is controlled by the pulse width modulator 991 whose signal isinput at node 914 of switch 906. The pulse width modulator signalcontrols how long the output on pin the pole 908 is connected to thereference voltage and how long the pole 910 is connected to theamplifier output. The pulse width modulator signal can be varied toeffect how long the pulse width is on or off.

The switch output signal is input into a first resistor capacitornetwork comprised of a resistor 914 and a capacitor 918 which averagesthe signal to produce an effective voltage. In addition, a secondresistor capacitor network comprised of a resistor 922 and a capacitor924 is tied to the output of the amplifier 903 and the switch 906. Itprovides additional filtering before input into the switch 906.

The LED Drive & Select Circuit

The pulse width modulator 991 of the microprocessor 990 controlsalternatively the amplification of the LED signal or the LED intensity,but not both. Typically upon initialization of the LED, the LEDintensity is started at its minimum value. The LED intensity isincreased until the signal is strong enough to see, and therefore beeasily detectable by the photodetector. If increasing the intensity toits maximum does not give an adequate signal, the signal strength of theLED may be increased by increasing the gain of the amplifier by eitherthe first or second variable gain amplifiers 600, 900.

The signal from the pulse width modulator on line 110 is input into afirst resistor 112 and a resistor capacitor network comprised ofresistors 114, 116 and capacitors 118, 120. The resistor capacitornetwork filters the signal. The signal on line 122 is proportional tothe duty cycle of the pulse width modulator. If, for example, the pulsewidth modulator is at its lowest setting the output would be equal to0.5 volts/256. This output signal on line 122 is used as a referencevoltage.

The reference voltage is input into pole 124 of switch 130 which selectswhether the reference voltage is used with respect to the infrared orred channel. If the red LED is to be turned on, then channel 124 will beselected and channel 128 will connect the noninverting input 132 ofamplifier 134 to the reference voltage. The output 126 of the amplifier134 is used to select the transistor, and thus the LED, the amplifier134 will drive.

There is an LED Drive Circuit for both the red signal and infrared LEDsignals. Although the two driving circuits are functionally equivalent,the red signal drive circuit is discussed by way of example. If pole 124is selected, the output of amplifier 134 will be connected to transistor136. Transistor 136 is turned on when amplifier 134 is on and isproducing sufficient current through resistor 138 that the voltageacross resistor 140 is equal to the reference voltage. This creates acurrent into the LED which is proportional to the reference voltage.

To turn off the LED, transistor 142 is turned on. Turning on transistor142, pulls up the inverting terminal of the amplifier 132 so that it ishigher than the reference voltage. This turns off the amplifier 142 andthus the LED.

Algorithm for Calculation of Ratio of Ratios

The microprocessor 990 uses the separated AC and DC components of themeasured signal to calculate the oxygen saturation content in the blood.The mathematical derivation for the Ratio of Ratios (RofR) is calculatedusing as a base the Beer-Lambert equation:

    I.sub.out =I.sub.in e.sup.-CL Sβ.sbsp.o.sup.+ 1-S!β.sbsp.r.sup.!

where I_(out) is the current out of the photodetector, I_(in) is thecurrent into the light emitting diode (the red LED for RofR for the redwavelength, the IR LED for RofR for the IR wavelength), C is theconcentration of the liquid (blood), L is the path length (between theLED and the photodetector), S is the Saturation, and β_(o) and β_(r) arematerial dependent constants.

By choosing two points in time you can eliminate the constant I_(in) andarrive at:

    I.sub.out (t.sub.o)=I.sub.out (t.sub.1)e.sup.-Cd Sβ.sbsp.o.sup.+ 1-S!β.sbsp.r!

where d=L(t_(o))-L(t₁) and is equal to the difference in path lengthbetween the two times t_(o) and t₁. In existing methods t_(o) ismeasured at the peak of the waveform and t₁ is measured at the valley ofthe waveform.

Dividing both sides by I_(out) (t₁) and taking the natural logarithmgives ##EQU1##

This is the function at one wavelength, so using both wavelengths anddividing the results yields the Ratio of Ratios. ##EQU2##

The Ratio of Ratios is typically calculated by taking the naturallogarithm of the ratio of the peak value of the infrared signal dividedby valley measurement of the red signal. The aforementioned value isthen divided by the natural logarithm of the ratio of the peak value ofthe red signal divided by the value of valley measurement of theinfrared signal.

Since the concentration "C" is the same for both wavelengths it cancelsout. In addition, the difference or "change" in path lengths is the samefor both channels, so it also cancels out. Thus the Ratio of Ratios maybe calculated according to the following formula. ##EQU3##

In the present invention, the Ratio of Ratios is determined usingderivatives. Assuming the change in path length is the same for bothwavelengths during the same time interval between samples, theinstantaneous change in path length (dL/dt) must also be the same forboth wavelengths. Thus the same Ratio of Ratios can be derived by takingthe derivative of I_(out). This can be shown mathematically by the sameprocess that was used to derive Ratio of Ratios:

    I.sub.out =I.sub.in e.sup.-CL Sβ.sbsp.o.sup.+ 1-S!β.sbsp.r.sup.!

Since de^(u) /dt=e^(u) du/dt and I_(in) is constant

    dI/dt out=I.sub.in e.sup.-CL Sβ.sbsp.o.sup.+ 1-S!β.sbsp.r.sup.! (-CdL Sβ.sub.o + S-1!β.sub.r !)

Dividing dI/dt out by I_(out) yields ##EQU4## therefore ##EQU5## usingtwo wavelengths and dividing gives ##EQU6## Since we know that thechange in the path length (dl/dt) is the same for both wavelengths, thiscancels out giving: ##EQU7## rearranging the formula gives: ##EQU8##where I_(out) is equal to the combined AC and DC component of thewaveform and dI/dt out is equal to the derivative of the and ACcomponent of the waveform. Thus the same equation for Ratio of Ratioscan be derived by taking the derivative of the Beer-Lambert function.Instead of using the previous method of calculating the Ratio of Ratiosbased on the natural logarithm of the peak and valley values of the redand infrared signals, the value RofR can be calculated based on thederivative value of the AC component of the waveform.

To calculate the Ratio of Ratios according to the derivative basedformula, a large number of sampled points along the waveform are usedinstead of merely the peak and valley measurements. A series of samplepoints from the digitized AC and AC+DC values for the infrared and redsignals are used to form each data point. A digital FIR filtering stepessentially averages these samples to give a data point. A large numberof data points are determined in each period. The period is determinedafter the fact by noting where the peak and valley occurs.

For the AC signal, a derivative is then calculated for each pair of datapoints and used to determine the ratio of the derivatives for red andIR. A plot of these ratios over a period will ideally result in astraight line. Noise from motion artifact and other sources will varysome values. But by doing a linear regression, a best line through aperiod can be determined, and used to calculate the Ratio of Ratios.

A problem with prior systems was DC drift. In prior methods, a linearextrapolation was performed between two consecutive negative peaks ofthe waveform. This adjusts the negative peak of the waveform as if theshift due to system noise did not occur. A similar correction can becalculated using the derivative form of the waveform. In performing thecorrection of the DC component of the waveform, we assume that the driftcaused by noise in the system is so much slower than the waveform pulsesthat the drift is linear. The linear change on top of the waveform canbe described by the function:

    f(x)=f(x)+mx+b

where m is equal to the slope of the waveform and B is equal to aconstant.

The linear change added to the waveform doesn't affect the instantaneousDC component of the waveform. However, the derivative of the linearchange will have an offset due to the slope of the interferring signal:##EQU9##

If we assume that the offset is constant over the period of timeinterval, then the Ratio of Ratios may be calculated by subtracting theoffsets and dividing: ##EQU10## where "y" and "y" are the originalvalues and m₁ and m₂ are the offsets.

Since the Ratio of Ratios is constant over this short time interval theabove formula can be rewritten as

    y-m.sub.2 /x-m.sub.1 =R

    y-m.sub.2 R (x-m.sub.1)

    y=Rx-Rm.sub.1 +m.sub.2

therefore

    y=Rx+(m.sub.1 +Rm.sub.1)

Since we have assumed m₁, m₂, and R are constant over the time interval,we have an equation in the form of y=mx+b where the m is the Ratio ofRatios. Thus, we do a large number of calculations of the Ratio ofRatios for each period, and then do the best fit calculation to the liney=Rx+b to determine the optimum value of R for that period, taking intoaccount the constant b which is caused by DC drift.

To determine the Ratio of Ratios exclusive of the DC offset we do alinear regression over the data points. In performing a linearregression, it is preferred to take points along the curve having alarge differential component, for example, from peak to valley. Thiswill cause the mx term to dominate the constant b. ##EQU11## where n=#of samples

j=sample #

x=I_(red) dI/dt IR

y=I_(IR) dI/dt RED

Prior sampling methods typically calculate the Ratio of Ratios bysampling the combined AC and DC components of the waveform at the peakand valley measurements of the waveform. Sampling a large number ofpoints on the waveform, using the derivative and performing a linearregression increases the accuracy of the Ratio of Ratios, since noise isaveraged out. The derivative form eliminates the need to calculate thelogarithm. Furthermore doing a linear regression over the sample pointsnot only eliminates the noise caused by patient movement of theoximeter, it also decreases waveform noise caused by other sources.

Although the invention has been explained by referenced to the foregoingembodiment, it should be understood that the above description is merelyillustrative and is provided for example only. Thus it should beunderstood that the invention is limited only in accordance with theappended claims.

What is claimed is:
 1. A method for determining a parameter of blood, comprising the steps of:transmitting first and second wavelengths of electromagnetic energy toward a tissue sample; detecting the first and second wavelengths of electromagnetic energy scattered by the tissue sample, thereby generating first and second analog signals corresponding to the first and second wavelengths, the first and second analog signals each having an AC and a DC component; separating the AC components of the first and second analog signals from the first and second analog signals to produce first and second separated AC components; computing the derivatives of the first and second separated AC components; taking a linear regression of a ratio of the derivatives of the separated AC components of the first and second analog signals for a plurality of sample points in a period; and computing the parameter of blood, the parameter of blood corresponding to the derivatives of the first and second separated AC components.
 2. The method of claim 1, further comprising the step of converting the separated AC components of the first and second analog signals into a first digital signal and a second digital signal, respectively.
 3. The method of claim 2, further comprising the step of amplifying the first and second analog signals to be in a range of an A/D converter used for the converting step, wherein the first analog signal is amplified at a different level then the second analog signal.
 4. The method of claim 2, further comprising the step of amplifying the separated AC components of the first and second analog signals to be in a range of an A/D converter used for the converting step.
 5. The method as recited in claim 2, wherein the amount of amplification of the separated AC components of the first and second analog signals is controlled by a pulse width modulator, wherein the length of the pulse of the pulse width modulator is proportional to the amount of amplification of the separated AC components.
 6. The method as recited in claim 2, further comprising the step of increasing the intensity of the first wavelength of electromagnetic energy such that the first analog signal is in a range of an A/D converter used for the converting step.
 7. The method as recited in claim 6, wherein the intensity of the first wavelength of electromagnetic energy is controlled by a pulse width modulator.
 8. The method as recited in claim 2, further comprising the step of converting the first and second analog signals into third and fourth digital signals, respectively.
 9. The method as recited in claim 8, wherein the step of computing the parameter of the blood is calculated using the relationship ##EQU12## where I_(out) λ₁ is equal to the third digital signal, I_(out) λ₂ is equal to the fourth digital signal, dI/dt outλ₁ represents the derivative of the separated AC component of the first analog signal, and dI/dt out λ₂ represents the derivative of the separated AC component of the second signal.
 10. The method as recited in claim 1, wherein the step of separating the AC components from the first and second analog signals includes,periodically charging first and second capacitors to approximately the value of the DC component of the first and second analog signals, respectively; and subtracting the DC component stored on the capacitors from the first and second analog signals, respectively.
 11. The method as recited in claim 1, wherein the derivatives of the AC components of the first and second analog signals are calculated from the change in value between two consecutive digital sample points in a period.
 12. The method as recited in claim 1, wherein the parameter of the blood is the oxygen saturation level.
 13. The method as recited in claim 1, wherein the first and second wavelengths of electromagnetic energy are in the infrared and red regions, respectively.
 14. The method of claim 1 further comprising the step of sensing an impedance corresponding to one of the first and second wavelengths of electromagnetic energy.
 15. An apparatus for measuring a parameter of the blood, comprising:at least two emitting means for emitting first and second wavelengths of electromagnetic energy toward a tissue sample; means responsive to the emitting means for detecting the first and second wavelengths of electromagnetic energy scattered by the tissue sample, the detection means producing first and second analog signals, each of said first and second analog signals having an AC and a DC component, the first analog signal corresponding to the first wavelength of electromagnetic energy and the second analog signal corresponding to the second wavelength of electromagnetic energy; means for separating the AC component of the first analog signal from the first analog signal to produce a first separated AC component and separating the AC component of the second analog signal from the second analog signal to produce a second separated AC component, wherein the means for separating includes a subtractor circuit; an analog to digital converter coupled to the means for separating, the analog to digital converter converting the first separated AC component into a first digital signal and the second separated AC component into a second digital signal, the analog to digital converter having an input voltage range; means for taking a linear regression of a ratio of derivatives of the first and second separated AC components for a plurality of sample points in a period, the means for taking a linear regression including a microprocessor which computes the parameter of blood from the first and second digital signals, wherein the value of the parameter of blood corresponds to the derivative of the first separated AC component and the derivative of the second separated AC component; and means for sensing an impedance corresponding to the at least one wavelength of electromagnetic energy.
 16. A method for determining a parameter of blood, comprising the steps of:transmitting at least one wavelength of electromagnetic energy through a sample; detecting the at least one wavelength of electromagnetic energy through the sample; generating at least one analog signal corresponding to the detected at least one wavelength, the at least one analog signal having an AC and a DC component; separating the AC component of the at least one analog signal from the at least one analog signal to produce a separated AC component; computing the derivative of the separated AC component; and computing the parameter of blood using the derivative of the separated AC component.
 17. The method of claim 16 wherein the at least one wavelength of electromagnetic energy comprises first and second wavelengths of light, and wherein the at least one analog signal comprises first and second analog signals, the first analog signal corresponding to the first wavelength of light and the second analog signal corresponding to the second wavelength of light.
 18. The method of claim 17, further comprising the step of taking a linear regression of a ratio of the derivatives of the separated AC components of the first and second analog signals for a plurality of sample points in a period.
 19. The method of claim 17, further comprising the step of converting the separated AC components of the first and second analog signals into a first digital signal and a second digital signal, respectively.
 20. The method of claim 19, further comprising the step of amplifying the first and second analog signals to be in a range of an A/D converter used for the converting step, wherein the first analog signal is amplified at a different level then the second analog signal.
 21. The method of claim 19, further comprising the step of amplifying the separated AC components of the first and second analog signals to be in a range of an A/D converter used for the converting step.
 22. The method as recited in claim 21, wherein the amount of amplification of the separated AC components of the first and second analog signals is controlled by a pulse width modulator, wherein the length of the pulse of the pulse width modulator is proportional to the amount of amplification of the separated AC components.
 23. The method as recited in claim 19, further comprising the step of increasing the intensity of the first wavelength of light such that the first analog signal is in a range of an A/D converter used for the converting step.
 24. The method as recited in claim 23, wherein the intensity of the first wavelength of light is controlled by a pulse width modulator.
 25. The method as recited in claim 19, further comprising the step of converting the first and second analog signals into third and fourth digital signals, respectively.
 26. The method as recited in claim 25, wherein the step of computing the parameter of the blood is calculated using the relationship ##EQU13## where I_(out) λ₁ is equal to the third digital signal, I_(out) λ₂ is equal to the fourth digital signal, dI/dt outλ₁ represents the derivative of the separated AC component of the first analog signal, and dI/dt outλ₂ represents the derivative of the separated AC component of the second signal.
 27. The method as recited in claim 17, wherein the step of separating the AC components from the first and second analog signals includes,periodically charging first and second capacitors to approximately the value of the DC component of the first and second analog signals, respectively; and subtracting the DC component stored on the capacitors from the first and second analog signals, respectively.
 28. The met hod as recited in claim 17, wherein the derivatives of the AC components of the first and second analog signals are calculated from the change in value between two consecutive digital sample points in a period.
 29. The method as recited in claim 17, wherein the first and second wavelengths of light are in the infrared and red regions, respectively.
 30. The method as recited in claim 16, wherein the parameter of the blood is the oxygen saturation level.
 31. A method for determining a parameter of blood, comprising the steps of:transmitting at least one wavelength of electromagnetic energy through a sample; detecting the at least one wavelength of electromagnetic energy through the sample; generating at least one analog signal corresponding to the detected at least one wavelength; computing the derivative of the analog signal; and computing the parameter of blood using the derivative of the analog signal.
 32. A method for determining a parameter of blood, comprising the steps of:transmitting first and second wavelengths of electromagnetic energy toward a tissue sample; detecting the first and second wavelengths of electromagnetic energy scattered by the tissue sample, thereby generating first and second analog signals corresponding to the first and second wavelengths, the first and second analog signals each having an AC and a DC component; computing the derivatives of the first an d second analog signals; taking a linear regression of a ratio of the derivatives of the first and second analog signals for a plurality of sample points in a period; and computing the parameter of blood, the parameter of blood corresponding to the derivatives of the analog signals.
 33. The method of claim 32, further comprising the step of subtracting a DC signal from the first and second analog signals.
 34. An apparatus for measuring a parameter of the blood, comprising:at least two emitting means for emitting first and second wavelengths of electromagnetic energy toward a tissue sample; means responsive to the emitting means for detecting the first and second wavelengths of electromagnetic energy scattered by the tissue sample, the detection means producing first and second analog signals, each of said first and second analog signals having an AC and a DC component, the first analog signal corresponding to the first wavelength of electromagnetic energy and the second analog signal corresponding to the second wavelength of electromagnetic energy; an analog to digital converter coupled to the means for separating, the analog to digital converter converting the first analog signal into a first digital signal and the second analog signal into a second digital signal; and means for taking a linear regression of a ratio of derivatives of the first and second analog signal for a plurality of sample points in a period, the means for taking a linear regression including a microprocessor which computes the parameter of blood from the first and second digital signals, wherein the value of the parameter of blood corresponds to the derivative of the first analog signal and the derivative of the second analog signal. 